Generally, a lithium ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte. As each of the positive electrode and the negative electrode, an electrode having an electrode active material layer is used. The electrode active material layer is usually formed by applying a composition containing an electrode active material, a conductive auxiliary agent, and a binder onto a current collector. In the lithium ion secondary battery, the electrode active material is an important factor involved in battery capacity, and graphite, silicon, or silicon oxide is used as a negative electrode active material, for example.
These negative electrode active materials have a function of occluding or releasing a lithium ion during charge and discharge, but have a problem that a lithium ion reacts irreversibly with the negative electrode active materials in initial charge, resulting in a decrease in battery capacity (discharge capacity). In order to avoid this problem, a treatment (pre-doping treatment) for doping a negative electrode active material layer constituting a negative electrode with a lithium ion in advance is performed before the initial charge. If the irreversible reaction is caused in advance by performing the pre-doping treatment, it is possible to suppress the irreversible reaction and generation of a by-product at the time of the initial charge thereafter.
The pre-doping treatment is performed by a method for bringing a lithium metal into contact with a negative electrode active material layer. In this case, it is known that an electrode interface film (SEI) is generated by decomposition of an electrolytic solution in addition to an irreversible active material such as lithium silicate (Li4SiO4) in the active material layer in direct contact with the lithium metal. The SEI functions as a lithium conductor capable of smoothly exchanging a lithium ion if the amount of the SEI is appropriate. However, in a case where the SEI is excessively generated, the SEI becomes a resistance and adversely affects battery characteristics. Patent Document 1 discloses a method for preventing generation of excessive SEI by rapid doping with a lithium ion by causing a negative electrode to face a Li source with a resistor interposed therebetween.